Conductive coil contact member

ABSTRACT

In a conductive coil contact member having at least one tapered end consisting of a plurality of turns of coil wire having a progressively smaller coil radius toward a free end thereof, the coil wire comprises a core wire and at least one highly electrically conductive layer formed over the core wire, a last turn of the coil wire at the free end having a smaller coil radius than would be possible by coiling the coil wire. Thus, the core wire is coiled to a smallest possible radius in the last turn, and the coil wire diameter is thereafter increased by forming layers formed by plating or other similar methods. The final result is that the last turn of the coil wire at the free end has a smaller coil radius than would be possible by coiling the coil wire. The reduction in the coil radius of the last turn contributes to the improvement in the positional accuracy of the free end of the tapered end of the conductive coil contact member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.09/909,523, filed Jul. 20, 2001, now U.S. Pat. No. 6,781,390 which isherein incorporated by reference for all purposes.

TECHNICAL FIELD

The present invention relates to a conductive coil contact membersuitable for use in contact probes and contact cards for testingsemiconductor devices and wafers, electric sockets for devices in suchforms as LGA (land grid array), BGA (ball grid array), CSP (chip sidepackage) and bare chip, and electric connectors. The present inventionalso relates to a contact probe head using a conductive coil contactmember.

BACKGROUND OF THE INVENTION

Conventionally, various forms of conductive contact members have beenused in electrically testing (open/short circuit tests, environmentaltests, burn-in tests and so forth) electroconductive patterns of printedcircuit boards and electronic components, and have been widely used incontact probes for testing wafers, electric sockets for semiconductordevices in such forms as LGA, BGA, CSP and bare chip, and connectors.

In applications to electric sockets for semiconductor devices, forinstance, there is a trend for using higher signal frequencies forsemiconductor devices, and frequencies in the order of hundreds of MHzhave become very common. In a socket for a semiconductor device whichoperates under such high frequency signals, the conductive contactmember which forms a part of the electrically conductive path for thedevice is required to have an even lower inductance and resistance. Forinstance, U.S. Pat. No. 6,043,666 issued Mar. 26, 2000 to T. Kazamadiscloses a coil spring having a closely wound conical end which servesas a contact member so that the contact member and compression coilspring are integrally combined, and the inductance and resistance arereduced.

In addition to the need to reduce the electric inductance andresistance, it is desired to ensure the positional accuracy of eachcontact member with respect to the object to be contacted particularlywhen a large number of such conductive contact members are arranged in asupport member to access a large number of points at the same time.

A conductive coil contact member allows the electric inductance andresistance to be minimized, but causes some difficulty in achieving thepositional accuracy of its tip with respect to the object to becontacted. The tip is formed by conically winding a coil wire, but thelast turn of the coil wire has a certain radius, and the position of thetip of the cone or the highest point of the cone could be anywhere alongthe last turn which is offset from the axial center of the cone by thisradius. In short, the position of the tip of a conically wound contactmember is offset from the axial center of the cone by a distance equalto the radius of the last turn of the coil wire.

Such an offset can be reduced by reducing the radius of the last turn,but it is difficult to reduce the radius of a turn beyond a certainlimit for a given diameter of the coil wire. If a coil wire having asmall diameter is used, it is possible to reduce the radius of the turn,but the desired resiliency or rigidity of the contact member may not beachieved, and the electric inductance and resistance inevitablyincrease.

U.S. Pat. No. 6,043,666 discloses a conductive coil contact memberhaving each coil wire end formed into a pin extending centrally in theaxial direction. Such an arrangement permits the positional accuracy ofeach tip to be increased without any limit in theory, but becomesimpractical when the size of the conductive coil contact member isreduced beyond a certain level.

BRIEF SUMMARY OF THE INVENTION

In view of such problems of the prior art, a primary object of thepresent invention is to provide a conductive coil contact member whichdemonstrates a low electric inductance and resistance while providing ahigh level of positional accuracy for the contact point thereof.

A second object of the present invention is to provide a conductive coilcontact member which provides a high level of positional accuracy forthe contact point thereof but is easy and economical to manufacture.

A third object of the present invention is to provide a conductive coilcontact member which demonstrates both mechanically and electricallyfavorable properties.

A fourth object of the present invention is to provide an improvedcontact probe using a conductive coil contact member.

According to the present invention, such objects can be accomplished byproviding a conductive coil contact member having at least one taperedend consisting of a plurality of turns of coil wire having aprogressively smaller coil radius toward a free end thereof, wherein:the coil wire comprises a core wire and at least one highly electricallyconductive layer formed over the core wire, a last turn of the coil wireat the free end having a smaller coil radius than would be possible bycoiling the coil wire.

Thus, the core wire is coiled to a smallest possible radius in the lastturn, and the coil wire diameter is thereafter increased by forminglayers formed by plating or other similar methods. The final result isthat the last turn of the coil wire at the free end has a smaller coilradius than would be possible by coiling the coil wire. The reduction inthe coil radius of the last turn contributes to the improvement in thepositional accuracy of the free end of the tapered end of the conductivecoil contact member.

According to a preferred embodiment of the present invention, aplurality of layers are formed over the core wire, the layers includingat least one highly electrically conductive layer, made of such materialas silver, copper and alloy of such a material, and at least one layerhaving a favorable mechanical property such as nickel. The outer layermay consist of material which is resistant to corrosion and oxidizationsuch as gold, rhodium and alloy of such material. The core wire may bemade of material having a favorable spring property such as steel.

The electrically conductive layer may either simply cover the coil wireor may continuously extend between adjacent turns of the coil wire sothat a favorable electric path extending in the axial direction may beachieved instead of a spiral path.

The present invention also provides a contact probe head, comprising: aholder consisting of at least one plate member and having at least oneholder hole extending across a thickness of the plate member; and aconductive coil contact member received in the holder hole, the coilcontact member having a first tapered coil end consisting of a pluralityof turns of coil wire having a progressively smaller coil radius towarda free end thereof, an intermediate coil spring potion including acoarsely wound section, and a second coil end which also consists of atapered coil end consisting of a plurality of turns of coil wire havinga progressively smaller coil radius toward a free end thereof, the freeend of the second tapered coil end being soldered to a terminal of anexternal base board.

The soldering connection for the second tapered end ensures both thepositional stability and favorable electric property of the conductivecoil contact member. Because of the conical shape of the second taperedend, it is possible to ensure a stable connection even if the amount ofapplied solder is small owing to the small cross sectional area of thefree end of the second tapered end, and prevent the solder from reachingthe compression coil spring portion and impairing the spring functionthereof owing to the capability of the second tapered end to retain apool of solder inside the tapered turns of the coil wire.

Similar results can be achieved if the second coil end consists of aplurality of turns of coil wire wound radially inwardly substantially ina plane perpendicular to an axial line of the coil contact member so asto define a substantially flat end surface, instead of a tapered end.

In such a contact probe head, for the ease of soldering work and otherassembly processes, it is desirable to be able to retain each conductivecoil contact member in the corresponding holder hole while allowing theconductive coil contact member to be installed in the holder holewithout any substantial difficulty. To achieve this goal, according to acertain aspect of the present invention, the contact probe head of thepresent invention may comprise a holder consisting of at least one platemember and having at least one holder hole extending across a thicknessof the plate member; a conductive coil contact member received in theholder hole; and means for resiliently retaining the contact member inthe holder hole.

The retaining means may comprise a section of the conductive coilcontact member having a slightly larger coil outer diameter than acorresponding inner diameter of the holder hole, or a plurality ofradial projections provided in a part of the holder hole such as an openend thereof, a circle defining by inscribing free ends of the radialprojections being slightly smaller than a corresponding coil outerdiameter of the conductive coil contact member. Alternatively, theretaining means may comprise a plurality of radial projections extendingover a substantial length of the holder hole on an inner circumferentialsurface of the holder hole, and a section of the conductive coil contactmember having a slightly larger coil outer diameter than a circledefining by inscribing free ends of the radial projections.

BRIEF DESCRIPTION OF THE DRAWINGS

Now the present invention is described in the following with referenceto the appended drawings, in which:

FIG. 1 is a sectional side view of a conductive coil contact memberembodying the present invention;

FIG. 2 is a enlarged fragmentary sectional view of an upper end of theconductive coil contact member shown in FIG. 1;

FIG. 3 is a schematic side view showing the conventional conductivecontact member;

FIG. 4 is a view similar to FIG. 2 showing an alternate embodiment ofthe present invention;

FIGS. 5 and 6 are fragmentary sectional views showing a lower conicalsection of the conductive coil contact member to explain the reasons forthe high level of tolerance on the amount of solder;

FIG. 7 is a sectional side view of a contact probe head embodying thepresent invention;

FIG. 8 is a view similar to FIG. 7 showing the embodiment shown in FIG.7 in actual use;

FIGS. 9 to 13 are views similar to FIG. 7 showing different embodimentswhich are modified from that shown in FIG. 7;

FIG. 14 is a sectional side view showing yet another embodiment of thecontact probe head according to the present invention;

FIG. 15 is a view similar to FIG. 14 showing the mode of assembling theembodiment illustrated in FIG. 14;

FIG. 16 is a view similar to FIG. 14 showing yet another embodiment ofthe contact probe head according to the present invention;

FIG. 17 is a view similar to FIG. 16 showing the mode of assembling theembodiment illustrated in FIG. 16;

FIG. 18 is an end view as seen in the direction indicated by arrow XVIIIin FIG. 17;

FIG. 19 is a view similar to FIG. 14 showing yet another embodiment ofthe contact probe head according to the present invention;

FIG. 20 is a view similar to FIG. 19 showing the mode of assembling theembodiment illustrated in FIG. 19; and

FIG. 21 is a sectional view taken along line XXI—XXI of FIG. 20.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a sectional view of a conductive coil contact member 1embodying the present invention. The conductive coil contact member 1comprises a cylindrical coil spring portion 1 a wound at a prescribedpitch in an intermediate part thereof and a pair of conical electrodeportions 1 b and 1 c each consisting of a closely wound portion andhaving a small diameter end facing outward. The lower electrode portion1 c as seen in the drawing is soldered to a circuit terminal 2 a of aprinted circuit board 2. An IC chip 3 is placed opposite the upperelectrode portion 1 b.

An electric connection can be established by moving the chip 3 downwardas seen in the drawing until the upper electrode portion 1 b comes intocontact with a pad 3 a of the IC chip 3, and the coil spring portion 1 ais resiliently compressed. The tip of the electrode portion 1 b can bethus engaged with the pad 3 a with a certain resilient force so that afavorable electric contact can be ensured between the electrode portion1 b and pad 3 a with an adequate contact pressure.

According to the illustrated conductive contact member 1, the two coilends are formed by winding a coil wire 5 into a conical shape with theirpointed ends facing away from each other. As shown in FIG. 2, the coilwire 5 consists of a core wire 4 and three plated layers 5 a, 5 b and 5c formed over the surface of the core wire 4. In other words, thediameter d₂ of the coil wire 5 is substantially greater than thediameter d₁ of the core wire 4. This coil wire 5 is formed by coilingthe core wire 4 into a conical shape, and the last turn on the smalldiameter end is formed so as to have as small a radius as possible forthe given wire diameter of the core wire 4. The three plated layers 5 a,5 b and 5 c are formed one after another after the core wire 4 is coiledto a prescribed shape. The plated layers can be formed any known methodsof forming such layers, and such method may include, not exclusive,electroplating, electroless plating, dipping, sputtering, CVD, PVD andthermal spraying. As can be readily appreciated, as the wire diameterincreases, the minimum radius that can be given to a turn increases.This radius R achieved by using the core wire 4 having the wire diameterd₁ is therefore substantially smaller than that would be achieved byusing a core wire having the diameter d₂.

When the conical electrode portion is brought into contact with anobject, for instance, having a flat contact surface, the point ofcontact may be anywhere along the last turn of the coil. In other words,the positional accuracy of the contact point is no less than the radiusR of the last turn of the coil at the coil end, and, therefore, can beimproved by reducing the radius R.

For instance, when the wire diameter of the core wire 4 is 110 μm, it isvery difficult to reduce the coil radius below 140 μm, but a coil radiusof 115 μm was achieving by using a coil wire having a wire diameter of90 μm. Therefore, the contact point was brought closer to the axialcenter C by 25 μm.

In case of a conductive contact member in the form of a needle member11, an error in the positional accuracy of the contact point can bereduced substantially without any limit as shown in FIG. 3. However, incase of a conductive contact member in the form of a coil spring, thecoil end may be shaped into a conical shape to minimize the positionalerror, but the error cannot be reduced any more than the radius of thelast turn of the coil. The error could be reduced by forming the coilend so as to place the wire end at the axial center, but it becomesincreasingly difficult as the coil diameter gets smaller, and, even ifit is possible, increases the manufacturing cost.

According to the foregoing embodiment, the conductive coil contactmember was coiled so as to achieve a small radius for the last turn ofthe coil wire by using a core wire having a small diameter. Therefore,the contact point can be brought close to the axial center of theconductive coil contact member, and the positional error in the contactpoint can be minimized. According to a presently preferred embodiment ofthe present invention, the core wire is made of a material having afavorable mechanical property such as steel, and at least one of theplated layers 5 a, 5 b and 5 c is made of a low electric resistancematerial.

When the coil wire is coiled into a conical shape, any radial gapbetween adjacent turns causes an excessive axial deflection of the coilspring so that the coil wire should be coiled in a conical shape whileensuring a certain radial overlap between adjacent turns of the coilwire. Therefore, if the wire diameter is small, the coil wire must becoiled into a conical shape having a relatively small cone angle, andthis increases the axial length of the conical part of the coil spring.This also increases the length of the electrically conductive path, andcompromises the compact design of such a conductive coil contact member.When the effective wire diameter is increased by forming plated layers 5a, 5 b and 5 c, the radial overlap between the adjacent turns of thecoil can be ensured while allowing a relatively large cone angle or arelatively small length for the conical part of the conductive coilcontact member,

According to a particularly preferred embodiment of the presentinvention, the lower plated layer consists of silver or copper having alow electric resistance, the middle layer consists of nickel whichprevents the diffusion of the lower layer, and the upper layer consistsof gold or rhodium having a resistance to oxidization and corrosion. Thenickel layer also contributes to the improvement in the mechanicalproperty such as resiliency of the conductive coil contact member.

The number of plated layers may be selected at will, and a single platedlayer may also be employed to implement the concept of the presentinvention. The thickness of each layer may be selected at will dependingon the nature of the selected material, and the particular applicationof the conductive coil contact member. For instance, silver or copperwhich has a lower electric resistance and is less expensive than goldcan be formed into a thick plated layer which may be in the order of afew μm to tens of μm in thickness. The plated layer additionallycontributes to the reduction in the contact resistance, improvement inmechanical properties, improvement in durability and preventingoxidization. The plating may be performed to the core wire before it iscoiled, or after it is coiled. The plated layer or layers typicallyhamper the coiling of the coil wire into a turn having a small radiusless than would a solid coil wire or core wire having a comparablediameter as the plated coil wire. It is also possible to perform theplating both before and after it is coiled. FIG. 4 shows the case wherethree plated layers 5 a, 5 b and 5 c are formed after the core wire 4has been coiled to a prescribed conical shape. Because the adjacentturns of the coil wire in the electrode portion 1 b are joined to oneanother by the plated layers, the electrode portion 1 b behaves, atleast electrically, as if it were made of a solid conical shell.

In this conductive coil contact member 1, the electrode portion 1 c issoldered to the circuit terminal 2 a of the circuit board 2 a. Thissoldering eliminates any contact resistance which inevitably arisesbetween two mutually abutting parts, and provides an accuratepositioning of the conductive coil contact member. It may also eliminatethe need for an additional fastening means such as threaded bolts.

Referring to FIGS. 5 and 6, this embodiment provides a high level oftolerance for the amount of solder that is used for securing theelectrode portion 1 c to the circuit terminal 2 a. Because the free endof the conical electrode portion 1 c has a small diameter, it can besoldered to the circuit terminal 2 a even when a very small amount ofsolder is used. Conversely, if a larger amount of solder is used, thesolder rises upward along the coil wire owing to the wettability of thecoil wire (or the capillary action) with respect to the solder, butbecause of the conical shape of the electrode portion, the solder ispooled inside the electrode portion 1 b. This latter tendency opposesthe tendency of the solder to rise upward along the electrode portion 1b. This is desirable because if the solder reaches the coil springportion 1 b, the resiliency of the conductive coil contact member 1would be impaired. In case of a cylindrical electrode portion or aconical electrode portion having a larger radius at its free end, thereis a less tendency for the solder to be pooled inside the electrodeportion, and the solder has a greater tendency to rise along theelectrode portion. In short, achieving a small radius for the free endof the conical electrode portion provides the advantage of increasingthe tolerance for the amount of solder that is used for attaching theconical electrode portion 1 c to a circuit terminal 2 a or the like.

At any event, according to this embodiment, it is possible to cover thesurface of the electrode portion 1 c with solder substantially in acontinuous manner without requiring any accurate control of the amountof solder so that the contact resistance that may be present betweenadjacent turns of the coil wire can be substantially reduced owing thesolder layer providing an alternate electrically conductive path, andthe electric signal is allowed to be conducted substantially along theaxial direction of the conical electrode portion 1 c, instead of along aspiral path. This obviously contributes to the reduction in the electricinductance and resistance of the conductive coil contact member 1.

FIG. 7 shows a contact probe head using such a conductive coil contactmember. This contact probe head comprises a holder formed by a pair ofinsulating plate members 7 and 8 integrally joined one over the otherusing threaded bolts, bonding agent or the like (not shown in thedrawing), and the insulating plate members 7 and 8 are formed withthrough holes 9 and 10, respectively, which are aligned with each otherand jointly form a holder hole. Only one set of through holes 9 and 10are shown in the drawings, but a large number of such sets of throughholes are typically formed in the holder. These through holes 9 and 10receive a conductive coil contact member 1 as described above.

The upper through hole 9 as seen in the drawing comprises a largediameter portion 9 a extending from the lower end over a large part ofthe thickness of the upper insulating plate member 7, an upper smalldiameter portion 9 b having a relatively small length at an upper openend of the through hole 9, and a tapered portion 9 c connecting the twoportions in a smooth manner. The coil spring portion 1 a of theconductive coil contact member 1 having a cylindrical profile isreceived in the large diameter portion 9 a, and the tapered portion 9 cengages a base end of the conical electrode portion 1 b of the contactmember 1 and restricts the outward movement of the contact member 1 tosuch an extent as to permit a free end of the upper conical electrodeportion 1 b to project out of the small diameter portion 9 b by aprescribed length and the axial line of the conductive coil springmember 1 is automatically aligned with the axial center C of the throughhole 9.

The lower through hole 10 comprises a small diameter portion 10 a facingthe large diameter portion of the through hole 9 in the upper insulatingplate 7, and a tapered portion 10 b having a small diameter endconnected to the small diameter portion 10 a and a large diameter endopening at the lower surface of the lower insulating plate member 8. Theannular shoulder defined between the two through holes 9 and 10 engagesthe lower conical electrode portion 1 c to restrict the downwardmovement of the electrode portion 1 c. Thus, the contact member 1 isretained in the support hole jointly defined by the two through holes 9and 10 in a pre-stressed state with the free ends of the upper and lowerconical electrode portions projecting from the corresponding openings ofthe support hole each by a prescribed length.

The holder assembly consisting of the two insulating plate members 7 and8, is placed over the circuit board 2, and is joined thereto by threadedbolts, a bonding agent or the like. The free end of the lower electrodeportion 1 c which resiliently abuts a circuit terminal 2 a of thecircuit board 2 is soldered thereto as indicated by W. The lower end ofthe tapered portion 10 b is so dimensioned as to expose the circuitterminal 2 a even when the lateral relative positioning between theholder and the circuit board 2 is not perfect.

FIG. 8 shows the assembly shown in FIG. 7 when it is applied to a pad 7a of an IC chip 7 which is desired to be tested. The assembly is movedtoward the IC chip 7, and retained at a prescribed short distancetherefrom by a fixture not shown in the drawing. Thus, the free end ofthe upper electrode portion 1 b engages the pad 7 a with a prescribedloading, and an adequate contact pressure is established between theelectrode portion 1 b and the pad 7 a.

FIG. 9 shows a second embodiment of the present invention in which theparts corresponding to those of the previous embodiment are denoted withlike numerals. In this embodiment, the through hole 9 of the insulatingplate member 7 facing away from the circuit board 2 is not differentfrom that of the previous embodiment, but the through hole 11 formed inthe lower insulating plate 8 on the side of the circuit board 2 consistsof a small diameter portion 11 a similar to that shown in FIG. 7 and alarge diameter portion 11 b provided on the side of the circuit board 2.The large diameter portion 11 b is so dimensioned as to expose thecircuit terminal 2 a even when the lateral relative positioning betweenthe holder and the circuit board 2 is not perfect. The large diameterportion 11 a consists of a straight hole, and this simplifies the workof forming the hole.

FIG. 10 shows a third embodiment of the present invention in which theparts corresponding to those of the previous embodiments are denotedwith like numerals. In this embodiment, the through hole 9 of theinsulating plate member 7 facing away from the circuit board 2 is notdifferent from that of the previous embodiment, but the through hole 12formed in the lower insulating plate 8 on the side of the circuit board2 consists of a straight small diameter hole extending over the entirethickness of the insulating plate 8. This simplifies the work of formingthe hole, and contributes to the reduction of the manufacturing cost.

FIG. 11 shows a fourth embodiment of the present invention in which theparts corresponding to those of the previous embodiments are denotedwith like numerals. In this embodiment, the holder in this case consistsof a single insulating plate member 13 which is connected to the circuitboard 2. A through hole 14 passed across the thickness of the insulatingplate member 13 comprises a large diameter portion 1 c extending fromthe end adjoining the circuit board 2 over a large part of the thicknessof the insulating plate member 13, a small diameter portion 14 bextending over a short distance from the end facing away from thecircuit board 2, and a tapered portion 14 c which smoothly connects thelarge diameter portion 14 a and small diameter portion 14 b together.

According to this embodiment, the holder for retaining the conductivecoil contact member 1 can be simplified. Similarly as the embodimentillustrated in FIG. 7, the conical electrode portion 1 b resilientlyabuts the tapered portion 14 c of the through hole so that the axialline of the conductive coil spring member 1 can be automatically alignedwith the axial center C of the through hole 14, and the projectinglength of the electrode portion 1 b can be made uniform.

FIG. 12 shows a fifth embodiment of the present invention in which theparts corresponding to those of the previous embodiments are denotedwith like numerals. In this embodiment, the holder in this case consistsof a single plate member 15 which is placed closely over the circuitboard 2. A through hole 16 passed across the thickness of the insulatingplate member 15 comprises a large diameter portion 16 a extending fromthe end facing away from the circuit board 2 over a large part of thethickness of the insulating plate member 15, a small diameter portion 16b extending over a short distance from the end adjacent to the circuitboard 2, and a tapered portion 16 c which smoothly connects the largediameter portion 16 a and small diameter portion 16 b.

According to this embodiment, simply by soldering the lower electrodeportion 1 c to the circuit terminal 2 a, the lower electrode portion 1 cis secured to the circuit board 2 with the maximum diameter portion ofthe lower electrode portion 1 c engaged by the tapered portion 16 c ofthe through hole 16. Therefore, according to this embodiment, thestructure is highly simplified not only because the holder consists of asingle insulating plate member 15 but also because the conductive coilcontact member 1 secures the insulating plate member 15 to the circuitboard 2 without requiring any threaded bolts or a bonding agent. Thisreduces the number of component parts and manufacturing steps so thatthe manufacturing cost can be minimized.

FIG. 13 shows a sixth embodiment of the present invention in which theparts corresponding to those of the previous embodiments are denotedwith like numerals. In this embodiment, the holder in this case consistsof a single plate member 17 which is connected to the circuit board 2. Athrough hole 18 is passed across the thickness of the insulating platemember 17. The through hole 18 in this case consists of a straight holehaving a constant diameter which allows the coil spring portion 1 a ofthe conductive coil contact member 1 to be guided in the axialdirection. This embodiment simplifies the structure even further, andthe manufacturing cost can be reduced accordingly.

FIG. 14 shows a seventh embodiment of the present invention in which theparts corresponding to those of the previous embodiments are denotedwith like numerals. In this embodiment, the holder hole 14 has asubstantially identical shape as that of the embodiment illustrated inFIG. 11. The conductive coil contact member 1 is provided with a conicalelectrode portion 1 b at its upper end as shown in the drawing, and abase end of the electrode portion 1 b is engaged by the tapered portion14 c of the holder hole 14 so as to have its free end project out of thesmall diameter portion 14 b of the holder hole 14 by a prescribedlength. The conductive coil contact member 1 is additionally providedwith an intermediate part consisting of a compression coil springportion 14 c which is coaxially received in the large diameter portion14 a of the holder hole 14 with a certain radial play.

The lower end of the conductive coil contact member 1 comprises aclosely wound portion 1 d having a relatively small axial length andconnected to a lower end of the compression coil spring portion 1 a, anda flat end portion 1 c which is sharply reduced in diameter from theadjacent closely wound portion 1 d by turning the coil wire radiallyinward in a plane perpendicular to the axial line of the conductive coilcontact member 1. Therefore, the flat end portion 1 c is provided with arelatively planar end surface by which the conductive coil contactmember 1 abuts the circuit terminal 2 a. This planar end surface enablesthe conductive coil contact member 1 to stand upright on the circuitterminal 2 a and allows the soldering work to be performed in a stablemanner.

Also, the solder fills into the gap between adjacent turns of the coilwire over the entire end surface of the flat end portion 1 c of theconductive coil contact member 1 so that a mechanically stable solderingconnection can be achieved on the one hand, and a favorable and stableelectric connection can be established between the end surface of theconductive coil contact member 1 and the circuit terminal 2 a.

The closely wound portion 1 d is provided with a diameter which issomewhat greater than that of the compression coil spring portion 1 a,and is slightly greater than the inner diameter of the large diameterportion 14 a of the holder hole 14 so that when the conductive coilcontact member 1 is placed inside the holder hole 14, the closely woundportion 1 d is resiliently engaged by the inner circumferential surfaceof the large diameter portion 14 a, and retains the conductive coilcontact member 1 in the holder hole 14.

When assembling the conductive coil contact member 1 into the holderhole 14, the conductive coil contact member 1 is fitted into the holderhole 14 from the large diameter end of the holder hole 14 with theconical electrode portion 1 b first. The closely wound portion 1 dhaving a larger diameter than the large diameter portion of the holderhole 14 is resiliently engaged by the open end of the large diameterportion 14 a, but can be pushed into the large diameter hole 14 a byapplying an inwardly pressure owing to the radially inward elasticdeformation of the closely wound portion 1 d. When the conductive coilcontact member 1 is fully received in the holder hole 14, the conductivecoil contact member 1 is firmly held in the holder hole 14, and thisfacilitates the assembly work. For instance, the soldering work can beperformed with an electrode portion consisting of the flat end portion 1c facing downward. Thus, although the holder consists of a single platemember, the conductive coil contact member 1 is retained in the holderhole and prevented from being dislodged in either axial directionwithout complicating the structure of the holder.

FIG. 16 shows an eighth embodiment of the present invention in which theparts corresponding to those of the previous embodiments are denotedwith like numerals. In this embodiment which is similar to the previousembodiment illustrated in FIGS. 14 and 15, the conductive coil contactmember 1 consists of an upper conical electrode portion 1 b, anintermediate compression coil spring portion 1 a, and a lower flat endportion 1 c defining a flat coil end, and is not provided with a closelywound portion having a larger diameter than the inner diameter of theholder hole. Instead, the open end of the large diameter portion 14 a ofthe holder hole 14 is provided with four radially inwardly directedprojections 13 a to define a gap b slightly smaller than the outerdiameter of the compression coil spring portion 1 a between eachopposing pair of the projections 13 a as shown in FIG. 18.

Referring to FIG. 17, when placing the conductive coil contact member 1in the holder hole 14, the conductive coil contact member 1 is fittedinto the holder hole 14 from the large diameter end of the holder hole14 with the conical electrode portion 1 b first. The projections 13 aengage the outer circumferential surface of the conductive coil contactmember 1 and provide a certain resistance to the axial movement of theconductive coil contact member 1 in the holder hole 14. When theconductive coil contact member 1 is fully received in the holder hole14, the conductive coil contact member 1 is firmly held in the holderhole 14, and this facilitates the assembly work. Thus, this embodimentprovides similar advantages as the previous embodiment.

FIG. 19 shows a ninth embodiment of the present invention in which theparts corresponding to those of the previous embodiments are denotedwith like numerals. In this embodiment which is similar to the previousembodiment illustrated in FIGS. 16 to 18, the conductive coil contactmember 1 consists of an upper conical electrode portion 1 b, anintermediate compression coil spring portion 1 a, a large diameterportion 1 d consisting of about a single turn of the coil wire and alower flat end portion 1 c defining a flat coil end. The open end of thelarge diameter portion 14 a of the holder hole 14 is provided with threeaxially extending radially inwardly directed projections 13 b as shownin FIG. 21. The outer diameter of the large diameter portion 1 d of theconductive coil contact member 1 is slightly larger than the diameter ofa circle inscribed by the free ends of the projections 13 b.

Referring to FIG. 20, when placing the conductive coil contact member 1in the holder hole 14, the conductive coil contact member 1 is fittedinto the holder hole 14 from the large diameter end of the holder hole14 with the conical electrode portion 1 b first. The projections 13 bengage the outer circumferential surface of the large diameter portion 1d of the conductive coil contact member 1 and provide a certainresistance to the axial movement of the conductive coil contact member 1in the holder hole 14. When the conductive coil contact member 1 isfully received in the holder hole 14, the conductive coil contact member1 is firmly held in the holder hole 14, and this facilitates theassembly work. Thus, this embodiment provides similar advantages as theprevious embodiment.

The retaining force or resistance acting on the conductive coil spring 1in the embodiments illustrated in FIGS. 14 to 21 should be such that theconductive coil spring 1 is retained in the holder hole 14 against thegravitational force or slight shaking. It is preferred to retain theconductive coil spring 1 in a precisely coaxial relationship withrespect to the holder hole to ensure the positional accuracy of the freeend of the upper electrode portion 1 b. In the last embodiment, thethree projections 13 b contribute to the centering of the conductivecoil spring 1 in the holder hole 14 without hampering the resilientaction of the compression coil spring portion 1 a.

The coil wire 4 in the foregoing embodiments had a circular crosssection, but may also have rectangular or other cross sectional shapes.In the foregoing embodiments, only one of the electrode portions wassoldered to the opposing terminal or the like, but depending on theapplication, the other electrode portion may be soldered to an objectinstead, or even both the electrode portions may be soldered to thecorresponding objects. In the last case, the contact resistance can betotally eliminated, and the overall electric resistance can beminimized.

In the foregoing embodiments, the holder was formed by two insulatingplate members or a single insulating plate member, but it is alsopossible to use three or more insulating plate members to form a holder.In such a case, the various portions of the holder hole having differentinner diameters may be formed by a corresponding number of plate membersformed with mutually aligned but different sized straight holes. In sucha case, the work required for forming holes in the holder can besimplified.

Thus, according to the present invention, the free end of the electrodeportion consists of a turn having an extremely small diameter for thegiven diameter of the coil wire, the positional accuracy of the contactend of the electrode portion can be increased. The plated layer alsocontributes to the reduction in the electric resistance of theconductive coil contact member.

If one end of the conductive coil contact member is solder to an objectsuch as a terminal, the contact resistance can be avoided, and theoverall electric resistance can be reduced. The soldering also improvesthe positional stability of the electrode portion against environmentalchanges or other interferences, and this contributes to the positionalaccuracy of the free end of the opposite electrode portion. If the twoends are both soldered, the contact pressure can be entirely eliminatedand the positional stability can be even more enhanced.

The electrode portion of the conductive coil contact member which issoldered to an object such as a terminal may be formed by coiling thecoil wire radially inward in a plane perpendicular to the axial line ofthe conductive coil contact member so as to define a flat end surface.This allows the conductive coil contact member to stand upright byitself, and enables the soldering work to be performed in a stablemanner. Also, the flat end contributes to a favorable distribution ofsolder so as to achieve both mechanically and electrically stableconnection.

If the compression coil spring portion of the conductive coil contactmember is appropriately guided by the inner circumferential wall of theholder hole, the positional accuracy of the free end of the electrodeportion can be ensured.

By providing radial projections in the inner circumferential surface ofthe holder hole or providing a large diameter portion having a slightlylarger outer diameter than the inner diameter of the holder hole, theconductive coil contact member can be favorably retained in the holderhole so that the soldering work can be performed in a stable manner.

Although the present invention has been described in terms of preferredembodiments thereof, it is obvious to a person skilled in the art thatvarious alterations and modifications are possible without departingfrom the scope of the present invention which is set forth in theappended claims.

1. A method of making a conductive coil contact member comprising a coilwire formed into a coil having at least one tapered end consisting of aplurality of turns of said coil wire having a progressively smaller coilradius toward a free end thereof, comprising: coiling a core wire into acoil in such a manner that a last turn of said free end is given with asubstantially smallest possible radius for a given wire diameter of saidcore wire; and forming an electrically conductive layer over said corewire at least at said free end so as to increase a diameter of said coilwire, said increased coil wire diameter being substantially greater thanwould be possible for said coil wire to be coiled and given with saidsmallest possible radius.
 2. A method of making a conductive coilcontact member according to claim 1, where a plurality of layers areformed over said core wire, said layers including at least one highlyelectrically conductive layer and at least one layer having a favorablemechanical property.
 3. A method of making a conductive coil contactmember according to claim 2, where said highly electrically conductivelayer is made of a member selected from a group consisting of silver,silver alloy, copper and copper alloy.
 4. A method of making aconductive coil contact member according to claim 2, where said layerhaving a favorable mechanical property is essentially made of nickel. 5.A method of making a conductive coil contact member according to claim2, where said layers further include an outer layer made of a memberselected from gold, gold alloy, rhodium and rhodium alloy.
 6. A methodof making a conductive coil contact member according to claim 2, wheresaid core wire is made of steel.
 7. A method of making a conductive coilcontact member according to claim 1, where said layer continuouslyextends between adjacent turns of said coil wire.
 8. A conductive coilcontact member having at least one tapered end consisting of a pluralityof turns of coil wire having a progressively smaller coil radius towarda free end thereof, wherein: said coil wire includes a core wire and atleast one electrically conductive layer formed over said core wire aftersaid core wire has been coiled to a prescribed shape, a last turn ofsaid coil wire at said free end having as small a coil radius aspossible for the diameter of said core wire.
 9. A conductive coilcontact member according to claim 8, wherein the prescribed shape is aconical shape.